i i \Grand_Attack" | 2015/4/2 | 0:19 | page 1 | #1 i i Jupiter's Decisive Role in the Inner Solar System's Early Evolution Konstantin Batygin ∗ and Gregory Laughlin y ∗Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA, and yDepartment of Astronomy & Astrophysics, UCO/Lick Observatory, University of California, Santa Cruz, Santa Cruz, CA 95064, USA Submitted to Proceedings of the National Academy of Sciences of the United States of America The statistics of extrasolar planetary systems indicate that the de- 4+ planet 3 planet fault mode of planet formation generates planets with orbital peri- systems ods shorter than 100 days, and masses substantially exceeding that systems of the Earth. When viewed in this context, the Solar System is unusual. Here, we present simulations which show that a popular 2 planet formation scenario for Jupiter and Saturn, in which Jupiter migrates systems inward from a > 5 AU to a ∼ 1:5 AU before reversing direction, can explain the low overall mass of the Solar System's terrestrial plan- ets, as well as the absence of planets with a < 0:4 AU. Jupiter's inward migration entrained s & 10 − 100 km planetesimals into low- order mean-motion resonances, shepherding and exciting their orbits. The resulting collisional cascade generated a planetesimal disk that, evolving under gas drag, would have driven any pre-existing short- period planets into the Sun. In this scenario, the Solar System's ter- restrial planets formed from gas-starved mass-depleted debris that R remained after the primary period of dynamical evolution. 10 planetary dynamics j solar system j extrasolar planets Significance he Solar System is an unusual member of the galactic Mars planetary census in that it lacks planets that reside in Earth T single planet close proximity to the Sun. In this work, we propose that the Venus systems Mercury primordial nebula-driven process responsible for retention of Jupiter and Saturn at large orbital radii and sculpting Mars' low mass is also responsible for clearing out the Solar Sys- Jupiter Saturn Neptune Earth Mercury tem's innermost region. Cumulatively, our results place the Fig. 1. Orbital distribution of sub-Jovian extrasolar planets. A Solar System and the mechanisms that shaped its unique or- collection of transiting planet candidates with radii R < 5R⊕, de- bital architecture into a broader, extrasolar context. tected by the Kepler mission is shown. The radial distance away from the center of the figure represents a logarithmic measure of the planetary semi-major axis, such that the origin corresponds to the Sun's surface. The sizes of the individual points represent the physical radii of the planets. Further, the points are color-coded Introduction in accordance with multiplicity. The orbits of the terrestrial plan- A full understanding of the formation and the early evolu- ets are also shown. Despite observational biases inherent to the tion of the Solar System ranks among natural science's grand observed distribution (e.g. transit probability, detectability) that challenges, and at present, even the dominant processes re- work against detection of planets at increasing orbital radii, the sponsible for generating the observed planetary architecture raw contrast to our own Solar System is striking. remain elusive. Nonetheless, the past three decades have gen- erated remarkable progress (1), and critically, the discovery of thousands of extrasolar planets has placed the Earth and the other Sun-like, planet-bearing stars, our terrestrial region is Solar System into the broader context of the galactic planetary severely depleted in mass. census. Reserved for Publication Footnotes Perhaps the most important exoplanet-related discovery has been the realization that roughly half of the Sun-like stars in the solar neighborhood are accompanied by systems of one or more planets on low-eccentricity orbits with peri- arXiv:1503.06945v2 [astro-ph.EP] 31 Mar 2015 ods ranging from days to months, and masses falling in the 1 M⊕ < Mp < 50 M⊕ range (2, 3). This dominant popula- tion of planets (which often presents tightly packed, nearly co-planar multiple systems) contrasts sharply with the Solar System, whose inner edge is marked by Mercury's 88-day (0.4 AU) orbit (see Figure 1). An iconic example from the new planetary catalog is the Kepler-11 system, which encompasses 1See refs (7, 8) for an alternative view. at least six planets comprising more than ∼ 40 Earth masses 2A notable system within the resonant extrasolar population is GJ 876, where the inner planet is (4). In short, the exoplanetary surveys have revealed a hith- substantially less massive than the outer. In accordance with the picture of resonant transport de- lineated in ref. (18), this system likely failed to satisfy the conditions required for migration reversal erto unrecognized oddity of the Solar System. Relative to and decayed to a compact orbital configuration (19). www.pnas.org | | PNAS Issue Date Volume Issue Number 1{11 i i i i i i \Grand_Attack" | 2015/4/2 | 0:19 | page 2 | #2 i i A few related peculiarities are also evident within the inner Solar System. Specifically, cosmo-chemical evidence suggests A Jupiter’s orbital migration that while the fundamental planetary building blocks (plan- etesimals) formed within ∼ 1 Myr of the Sun's birth (5), the final assembly of the terrestrial planets occurred on a timescale of 100−200 Myr, well after the dispersal of the nebular gas (6). 2:1 resonance This is at odds with the inferred compositions of extrasolar outward scattering unswept Super-Earths, which are thought to have substantial gaseous disk atmospheres. Additionally, the exceptionally small masses of Mercury and Mars suggest1 that the terrestrial planets formed resonant transport out of a narrow annulus of rocky debris, spanning 0:7 − 1 AU orbital eccentricity (where 1AU is the mean distance between the Earth and the Sun) (9). Such a narrow annulus is at odds with so-called 100 km planetesimals minimum mass Solar Nebula (10, 11). Within the framework of a radially confined solid component of the inner Solar nebula, the inner edge of the annulus is en- semi-major axis (AU) tirely artificial. Indeed, at present there exists no compelling justification for its origin. A plausible explanation may stem B from the dynamical evacuation of solid material by a popula- tion of primordial close-in planets (12). We shall investigate this possibility further in this study. Unlike the inner edge of the annulus, a body of previous 2:1 MMR work has demonstrated that the outer edge can be naturally sculpted by inward-then-outward migration of Jupiter (13). 3:2 Within protoplanetary disks, long-range migration of giant resonant capture and transport planets results from tidal interactions with the nebula and 1:1 viscous transport (14). For single planets, orbital evolution - planetesimal period ratio outward scattering is typically inwards. However, the process of resonant lock- ing between two convergently migrating planets can lead to a Jupiter reversal of the migration direction (15). The process of resonant migration reversal for gap-opening time (years) planets (i.e. objects with M & MJup) is a well-understood result of planet-disk interactions, and only requires the outer planet to be somewhat less massive than the inner. To this Fig. 2. Orbital evolution of planetesimals embedded in the So- end, it is worth noting that all of the known mean-motion com- lar nebula, under the effects of a migrating Jupiter. As Jupiter moves inwards from 6 AU to 1:5 AU, planetesimals are swept up by mensurate pairs of giant planets that reside beyond a & 1 AU have the more massive object on the inside (17, 16), suggesting mean motion resonances (MMRs). Panel A of this figure shows the 2 increase in the planetesimal eccentricity associated with resonant that the operation of this mechanism is widespread . transport. Note that at the end of the Jupiter's trek, there exists Within the Solar System, it is inferred that Jupiter initially a strong enhancement in the planetesimal density at the Jovian 2:1 migrated inwards from its primordial formation site (presum- MMR. Panel B depicts the preferential population of Jupiter's in- ably 3 − 10 AU) to ∼ 1:5 AU, and subsequently reversed its terior MMRs. Each planetesimal in the simulation is color-coded evolutionary track as a consequence of locking into a 3:2 mean- in accord with its initial condition, and the resultant curves track motion resonance with a newly-formed Saturn. This special the orbital excursions of the small bodies as Jupiter's orbit shrinks. case of the generic resonant migration reversal mechanism is Jupiter's return to ∼ 5 AU is not modeled directly. In the presented informally referred to as the \Grand Tack" scenario (13). In simulation, we assumed a planetesimal size of s = 100 km. Similar addition to the aforementioned truncation of the inner solid figures corresponding to s = 10 km and s = 1000 km can be found in the SI. nebula, this putative sequence of events is attractive in that it naturally explains how the Solar System's giant planets avoided spiraling into Sun (18), accounts for the origins of same period ratio, leading to a decrease in the planetesimal compositional differences within the Asteroid belt (13), pro- orbit's semi-major axis (24). The most common commensura- vides a mechanism for delivery of water into the terrestrial bility at which capture occurs is 2:1, although numerous other region (20), and generates a compact orbital configuration possibilities exist. needed for the subsequent instability-driven orbital evolution In order for resonant interactions to be effective, the plan- of the outer Solar System (21, 22).